Optical monitoring in Dense Wavelength Division Multiplexing (DWDM) systems is deployed for detecting the presence or absence of wavelength channels and measuring the optical powers of the channels, using pilot tones as an economic means. Metro and long haul networks utilize Optical Amplifiers (OA) to increase the distance between nodes. Unfortunately the OAs, such as the Erbium Doped Fiber Amplifier (EDFA) and the Semiconductor Optical Amplifier (SOA), introduce cross-talk in the process of amplifying a specific optical channel, such that attenuated copies of tones (cross-talk) from other channels on the DWDM fiber are superimposed on the specific optical channel. The cross-talk tones indicate the presence of non-existent channels and introduce inaccuracies in power measurement. Undesirable spurious tones on any optical channel may cause poor Signal to Noise Ratio (SNR) on the specific channel and other optical channels via cross-talk in an OA, which makes accurate optical monitoring very difficult.
The problem of cross-talk among multiple channels that are amplified by a given OA is explained with the help of FIG. 1. Consider two channels corresponding to wavelengths λ1 and λ2 that are incident on the input of the OA 100. The amplitudes of the signals at time t1 are 6 units (u) and 6 u respectively. The gain of 1.5 produced by the amplifier results in an output of 9 u for each of the channels at time t1+Δt. Consider a time instant t2 at which the amplitude of λ1 has changed to 3 u, while the amplitude of λ2 is still 6 u. The expected output for λ1 is 4.5 u and the output for λ2 should remain unchanged at 9 u. However, due to Cross Gain Modulation (XGM), the gain of the amplifier is changed to 2 and the outputs at time t2+Δt for λ1 and λ2 become 6 u and 12 u respectively. Note that these outputs are achieved because the OA 100 attempts to maintain the same output power level (corresponding to 18 u) at instants t1+Δt and t2+Δt. Clearly, the additional 3 u appearing at λ2 is spurious and corresponds to cross-talk induced by the channel with the wavelength of λ1.
The “Link Method” described in articles by Srivastava, A. K. et al., “Fast-Link Control Protection of Surviving Channels in Multiwavelength Optical Networks”, IEEE Photonics Technology Letters, Vol. 9, pp. 1667–1669, December 1997 and by Chung H. S. et al., “Effects of Stimulated Raman Scattering on Pilot-Tone-Based WDM Supervisory Technique”, IEEE Photonics Technology Letters, Vol. 12, pp. 731–733, June 2000, has been used for cross-talk cancellation, which involves maintaining the input power of the EDFA constant by using an external modulated Continuous Wave (CW) source, that adds to system cost.
Another technique has been described by Kim H. K. and Chandrasekhar S., “Reduction of Cross-Gain Modulation in the Semiconductor Optical Amplifier by Using Wavelength Modulated Signal”, IEEE Photonics Technology Letters, Vol. 10, pp.1412–1414, October 2000, in which external multiple modulated CW sources have been used for cross-talk cancellation in SOAs, by maintaining the input power to the SOA constant. This technique requires an external CW laser source for each DWDM channel supported, thus increasing the system cost for each additional DWDM channel.
A drawback of these methods is that they require additional hardware that increases the cost of the system.
In technical papers by Freeman J., and Conradi, J., “Gain Modulation Response of Erbium-Doped Fiber Amplifiers”' IEEE Photonics Technology Letters, Vol.5, pp. 224–226, February 1993, and by Novak S. and Moesle, A., “Analytic Model for Gain Modulation in EDFAs”, Journal of Lightwave Technology, Vol.20, No.6, pp.975–985, June 2002, encoding low frequency tones on an optical channel via EDFA pump modulation has been described. These methods use encoding of a tone on a channel via EDFA pump modulation, but do not provide a reference to using the encoding tone to cancel cross-talk in optical amplifiers.
Encoding and decoding low frequency tones on an optical channel via intensity modulators has been described by Mohammad Fatehi et al. in U.S. Pat. Nos. 5,892,606, 5,801,863, and No. 5,745,274. These methods provide encoding of tones on a channel and removal of tones from the channel, but do not address the problem of channel cross-talk in optical amplifiers.
Accordingly, there is a need in the industry for the development of an improved method and system for cross-talk cancellation in optical amplifiers, which would be simple, inexpensive and provide precise and selective cancellation of cross-talk tones in various types of amplifiers.